A transparent conductive substrate has been conventionally used as an antistatic film, a transparent electrode for touch panels or solar cells, an electromagnetic shielding material, etc. Among them, the electromagnetic shielding material has been intensively studied and researched in the various applications, and used for the purpose of suppressing emission of various kinds of electromagnetic waves from electronic equipments such as domestic appliances, cellular phones, personal computers and televisions. In particular, there is a fear that electromagnetic waves emitted from flat panel displays such as plasma displays and liquid crystal displays induce malfunction of devices and also have adverse influences on human bodies when exposed thereto for a long period of time. The methods for eliminating adverse influences from an electromagnetic wave generating source to suppress emission of the electromagnetic waves are generally classified into the following two types, i.e., (1) a method in which an electronic circuit is improved and designed so as to suppress emission of electromagnetic waves from the electromagnetic wave generating source; and (2) a method in which the electromagnetic wave generating source is covered with an electromagnetic shielding material. The above method (1) is more suitable as long as emission of electromagnetic waves is well suppressed, because no surplus members are needed therein. However, since the method (1) requires complicated procedures for identifying the source from which the electromagnetic waves are generated and further improving and designing the electronic circuit, there are many cases where use of the method (2) is more efficient and desirable rather than the method (1). Under this circumstance, studies have been made to develop a suitable electromagnetic shielding material.
As to the above method (2), there have been conventionally proposed several methods for producing a transparent conductive substrate that can be used in the applications of the electromagnetic shielding material. For example, there is known the method of forming a grid-like metal wiring. In this method, a metal foil such as a copper foil is formed into the grid-like metal wiring by photolithographic process (for example, refer to Patent Document 1). However, the photolithographic process requires not only complicated procedures but also a special apparatus which results in high costs. Also, there is disclosed the process in which Pd fine particles as a plating catalyst are applied onto a substrate and then the thus coated substrate is subjected to plating (for example, refer to Patent Document 2). However, in the above process, it is required to keep the plating solution under careful control which also results in complicated procedures and further causes the problem concerning environmental burden. In addition, there is disclosed the method for producing a transparent conductive substrate in which a printing ink prepared from metal fine particles is printed on a substrate by screen printing method and then heat-treated to form a grid-like metal wiring thereon (for example, refer to Patent Document 3). However, this method has such a problem that the metal wiring produced by such a printing method fails to have a sufficiently narrow width capable of conforming to still higher resolution of future displays.
Meanwhile, when the transparent conductive substrate having the grid-like metal wiring is applied to flat panel displays, etc., an “anti-moire” property of the transparent conductive substrate is important. The moire means spots appearing when points or lines that are distributed in a geometrically regular manner are overlapped, and more specifically is such a phenomenon that striped patterns are observed on displays. For this reason, it has been pointed out that a metal wiring having a random network structure, for example, is preferably used in place of the grid-like metal wiring. As a method of producing a transparent conductive substrate having the above random network-shaped metal wiring without using a photolithographic method or a printing method, there is disclosed the method in which a dispersion solution of metal fine particles is applied on a substrate and then dried to form a random network structure of the metal fine particles by utilizing a self-organizing phenomenon of the metal fine particles (for example, refer to Patent Document 4). This production method is considered to be more excellent in view of costs and reduction of environmental burden.
Also, as the above electromagnetic shielding material, there are mentioned metal foils such as aluminum foils, copper foils and iron foils, conductive cloths prepared by plating a polyester fiber with copper or nickel, metal shielding screens produced by arranging metal wires such as stainless steel wires and copper wires on a transparent substrate in a grid-like manner, transparent conductive substrates produced by printing a conductive paste containing a silver filler on a substrate in a grid-like manner, transparent conductive substrates produced by laminating ITO (oxide of indium and tin) on a substrate by vapor deposition, etc.
On the other hand, in recent years, the above electromagnetic shielding materials are required to have not only an electromagnetic shielding property but also a visibility for rendering objectives to be shielded viewable from outside. This is because there is a demand for subjecting electronic equipments to visual inspection to inspect a substrate or a wiring disposed therewithin from an outside of a housing thereof after being assembled.
To meet the above demand, there has been proposed the method in which a housing prepared from a transparent resin is attached with an optically transparent electromagnetic shielding material selected from the above-mentioned electromagnetic shielding materials to ensure a good visibility thereof.
Examples of the electromagnetic shielding material having an optical transparency include conductive cloths prepared by plating a polyester fiber with copper or nickel, metal shielding screens produced by arranging metal wires such as stainless steel wires and copper wires on a transparent substrate in a grid-like manner, transparent conductive substrates produced by printing a conductive paste containing a silver filler on a substrate in a grid-like manner, transparent conductive substrates produced by laminating ITO on a substrate by vapor deposition, etc.
In addition, when reduction of costs is intended, rather than the above method of attaching the electromagnetic shielding material to the housing of electronic equipments after completion of assembly thereof, it is apparently preferred from industrial viewpoints to use the method in which the transparent resin substrate as a raw material of the housing is first imparted with an electromagnetic shielding property, and then the resulting electromagnetic shielding transparent resin substrate is subjected to molding process to produce the housing.
However, in the case where the above electromagnetic shielding material is subjected to molding process in place of the transparent resin substrate as a raw material of the housing, or in the case where the electromagnetic shielding material is attached or laminated onto a molding transparent resin substrate and then the resulting composite or laminate is molded and formed into a shape of the housing, there tends to arise such a problem that bent corner portions of the housing suffer from defects which therefore results in difficulty in production of the housing.
More specifically, when using the metal shielding screen or the transparent conductive substrate produced by printing a conductive paste containing a silver filler on a substrate, the wirings formed by the metal wire or silver filler tend to be broken at the bent portions of the housing, so that it may be difficult to ensure a current flow through an entire portion of the housing. As a result, it is required to connect the respective current-flowing portions to ground when the housing is grounded, or to subject the housing to a secondary treatment to form a bridge between the current-flowing portions to ensure a current flow through an entire portion of the housing.
In addition, when using the transparent conductive substrate produced by applying ITO onto a substrate by vapor deposition, the resulting ITO thin film tends to suffer from cracks at bent portions thereof, thereby failing to ensure a current flow through an entire portion of the housing.
On the other hand, there is disclosed a transparent conductive film having a random network structure formed from metal fine particles (Patent Documents 5 and 6). It is expected that the transparent conductive film is used as an electromagnetic shielding material having an excellent visibility because of a high optical transparency and a high electrical conductivity thereof.    Patent Document 1: Japanese Patent Application Laid-open (KOKAI) No. 2001-217589    Patent Document 2: Japanese Patent Application Laid-open (KOKAI) No. 2005-203484    Patent Document 3: PCT Pamphlet WO 2005/115070    Patent Document 4: Japanese Patent Application Laid-open (KOKAI) No. 2006-32197    Patent Document 5: Japanese PCT Application Laid-Open (TOKUHYO) No. 2005-530005    Patent Document 6: Japanese Patent Application Laid-open (KOKAI) No. 2007-234299